Hydrides of silicon, germanium and tin are commercially valuable compounds that have been long known, but difficult to make. Hydrides of silicon, for example, may be hydrolized in aqueous acidic solutions to form siloxanes which are useful in waterproofing fabrics.
In the past, these hydrides of silicon, germanium and tin have been prepared by a number of reduction processes which are exemplified in the patents discussed in the following paragraphs. Each of the processes has deficiencies which are overcome by the instant process.
U.S. Pat. No. 4,295,986 to Gordon teaches the reduction of silicon, germanium and tin halides using an alkali metal hydride catalytically activated by a solution of an alkali borohydride in a suitable polyether solvent. Care must be exercised to avoid depletion of the alkali metal hydride since, if it is consumed, reduction will continue with the alkali borohydride in solution causing borane to build up in concentration finally liberating spontaneously flammable diborane gas and/or causing undesirable rearrangements of some reduction products.
U.S Pat. No. 3,099 672 to Cooper et al. teaches a process for the reduction of organo halogen silanes, organo alkoxy silanes, and silicon tetrachloride with sodium hydride at temperatures of from 175.degree. to 350.degree. C. This process is restricted to cases where reactants and products are stable at the high temperatures required. It also involves excessive energy consumption, thus raising the cost of making silicon hydrides. Moreover, with the higher temperatures, additional problems of control and the presence of undesirable by-products are injected into the process.
U.S. Pat. No. 3,535,092 to Chalk teaches a process that can be run at lower temperatures than the Cooper et al. process. Chalk teaches a reduction of halogen-contining silicon compounds with sodium hydride in the presence of an aprotic solvent selected from the class hexaalkylphosphoramides, octaalkylpyrophosphoramides, tetraalkylureas, and mixtures thereof. This class of solvents is expensive and suspected to be carcinogenic.
U.S. Pat. No. 3,043,857 to Jenkner teaches reduction of halides of silicon, germanium and tin with alkali metal hydride in the presence of metal organic compounds of boron, gallium, and aluminum at 40.degree. to 180.degree. C. in an inert organic solvent. Many of the catalysts of this process are pyrophoric, thus posing a fire risk. Also, separation of the product can be extremely difficult due to the close boiling points of the product and some of the metal organic compounds of boron, gallium and aluminum that are used. Jenkner also mentions the nonpyrophoric alkoxy and phenoxy compounds of boron and aluminum as catalysts, but these are much less active than the pyrophoric alkyls and require high temperatures and concentrations to be effective.
U.S Pat. No. 3,496,206 to Berger teaches reduction of organo silicon halide with alkali metal hydride in the presence of alkyl aluminum halide at temperatures of -20.degree. to 150.degree. C. in a substantially inert organic solvent. This process is similar to that of Jenkner but uses a less volatile catalyst form.
The process of the instant invention comprises reacting halogen compounds of silicon, germanium and tin with lithium hydride in the presence of tetrahydrofuran (THF) as a solvent at temperatures between 25.degree. C. and 67.degree. C. Where the reduction products have a boiling point less than that of THF, stoichiometric amounts of lithium hydride can be used. For products with boiling points greater than that of THF, an excess of lithium hydride is required. Likewise, the volumetric efficiency of the reaction is affected by the boiling point of the desired product. Where the boiling point is less than that of THF, the volumetric efficiency is excellent, that is, about 0.6 to 1.7 moles of THF per mole of the starting organohalo compound is required. Where the boiling point of the desired product is greater than that of THF, an eight to ten fold excess of THF is required. This probably is the result of the product which is a liquid at reaction conditions accumulating in the reaction vessel as the reaction progresses and affecting the overall polarity of the mixture. Where the product has a lower boiling point than THF, it is a gas under reaction conditions and distills out of the reaction vessel.
This process has several advantages over the processes reported in the art. The solvent is a less-expensive, noncarcinogenic solvent; the safety hazards associated with using pyrophoric materials are avoided; it is not critical to maintain an excess of the alkali hydride; and high temperatures are avoided. The process also has high yields, is not restricted by long reaction times, can be run batchwise or continuously, and yields a product that is easily separated from the reaction mixture.